It has been estimated that at least 1.3 trillion barrels of oil and gas exist below the sea floor. Deep water drilling occurs at depths generally greater than 1,000 feet. For example, in Brazil's ultra-deep oil fields, producers have to go through 7,000 feet of water, over 10,000 feet of sand and rocks as well as another 6,000 feet of salt, to extract the oil. This presents a variety of technical challenges. In addition, reaching the oil and gas is dangerous and expensive. For example, in 2007, the day rate alone for renting a drilling rig was over $500,000. Offshore oil exploration traditionally involves the repeated firing of large underwater air guns, fired from a ship, to create seismic pulses. These pulses are used for sonar-like mapping of rock layers beneath the seabed. The cost of operating such a ship is extremely high. Additionally, it is believed that these practices cause substantial harm to marine mammals, especially whales.